The invention relates to a device for optoelectronic interface measurement and refractometry in liquids.
Interface measurement and refractometry and/or display devices of this kind are used for example to measure fullness or to monitor the liquid boundary layer between two immiscible liquids of different densities in explosion-prone tanks, e.g. liquefied gas tanks or containers of solvents or the like, with the level of a liquid interface being determined from the change in the refractive index at the transition between the liquids. In this case, the electrical or electronic control devices must be located outside the explosion-prone area. The signal that monitors the liquid level is therefore generated in the explosion-prone area exclusively by optical means, then transmitted and modulated, with electrical evaluation being performed at a location remote from the liquid container.
These interface or refractive index display or measuring devices have a sensor located above the surface of the liquid, to which light is fed from at least one light source through optical waveguides and whose interface, as long as it remains above the liquid medium, partly reflects the coupled light due to total reflection. The reflected light is in turn fed to a photodetector through an optical waveguide. However, if the sensing element, made of transparent glass or plastic, comes in contact with the surface of the liquid, a portion of the light striking the interface will fall below the total reflection angle as a function of the refractive index of the medium and be decoupled into the liquid, with only a small amount reaching the photodetector.
Offenlegungsschrift DE 33 02 089 A1 teaches a system suitable for this purpose wherein the light is guided through multimode optical waveguides by total internal reflection.
One key disadvantage of this arrangement is that the signal deflection, i.e. the signal differential, between the value measured when the sensor is not wet and the value when it is wet is comparatively small. This means that many of the light modes guided in the optical waveguide and reflected many times at the inside wall of the waveguide escape from the boundary layer upon contact with air because they fall below the total reflection angle. Even when the sensing element is wetted by the liquid, disturbing reflections that reduce signal deflection can be expected, sharply reducing the potential sensitivity (difference in refractive index with respect to the liquid, change in degree of reflection).
The goal of the present invention is to provide an optoelectronic device of the type described, especially for monitoring explosion-prone liquids, said device avoiding the above noted disadvantages while ensuring maximum possible signal defection.